What is the best transport for a tourist in Berlin?

I think you should rent a bus for the group. This will make it easier to move all together, you will never get lost. It's just comfortable, because all the tours can take place with a guide just in your bus. And it's quite comfortable. I know that comfortable buses can be rented from this company https://bcs-bus.com/charter-bus-berlin They just provide such services. I think you can rent a bus for them without any problems.